1. Field of the Invention
The present invention is directed to a method and apparatus for minimizing the effects of drift on small scale metrology measurements, and more particularly, to a method and apparatus of correcting for a position shift from a tip-sample target location such as that which occurs during lock down of an air bearing stage used in a scanning probe microscope.
2. Description of Related Art
Several probe-based instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. Such measurements are often made on the nanoscale so positioning between the probe and sample is a challenge and often leads to corrupted data. Known systems lack the desired precision and, moreover, are susceptible to factors that compromise the ability to obtain reliable data.
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a sharp tip to make a local measurement of one or more properties of a sample. More particularly, SPMs typically characterize the surfaces of such small-scale sample features by monitoring the interaction between the sample and the tip of the associated probe assembly. By providing relative scanning movement between the tip and the sample, surface characteristic data and other sample-dependent data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
The atomic force microscope is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into direct or intermittent contact with a surface of the sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as an arrangement of strain gauges, capacitance sensors, etc. AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
Preferably, the probe is scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, for example, in Hansma et al. supra; Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs can be designed to operate in a variety of modes, including contact mode and oscillating flexural mode. In an oscillation “flexural mode” of operation the cantilever oscillates generally about a fixed end. One flexure mode of operation is the so-called TappingMode™ AFM operation (TappingMode™ is a trademark of the present assignee). In a TappingMode™ AFM, the tip is oscillated flexurally at or near a resonant frequency of the cantilever of the probe. When the tip is in intermittent or proximate contact with the sample surface, the oscillation amplitude is determined by tip/surface interactions. Typically, amplitude, phase or frequency of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. These feedback signals are then collected, stored, and used as data to characterize the sample. When measuring semiconductor samples, such as the trench capacitors discussed herein, a particular version of this oscillating mode known as deep trench (DT) mode, which employs a unique and costly tip, is used.
As metrology applications demand greater and greater throughput, and as the desirability of using SPM in a wide variety of applications requiring sub-micron measurements continues to grow, improvements to data acquisition using SPM have become necessary. Wafer analysis in the semiconductor industry is one key application. In general, chip makers want to measure structures (e.g., lines, vias, trenches, etc.) having critical dimensions (CDs) that are 90 nm and below. When analyzing these structures at such small scale, the corresponding measurements require uniformity control and must be able to accommodate high volume production environments. In this regard, one advancement has been in the area of automated AFMs which greatly improve the number of samples that may be imaged in a certain time frame by minimizing expert user tasks during operation. Instruments for performing automated wafer measurements are varied but AFM offers a unique solution by providing, for example, the ability to perform high-resolution multi-dimension (e.g., 3-D) imaging. Some instruments, like the Dimension X automated AFM offered by Veeco Instruments, have proven 200 mm and 300 mm automation platforms.
More particularly, two performance metrics to be considered when evaluating instruments used to make such measurements include throughput and repeatability. Throughput, in this case, typically is the number of wafers that may be imaged per unit time, and repeatability is the variation in results obtained from repeated measurements made on the same object under substantially identical conditions. These measurements most often must be proved prior to the tool being useful. A third issue concerns reproducibility, which is the variation that results when making the same measurement under different conditions. Reproducibility is important in that it determines whether the technique can accommodate condition variations when both positioning the wafer and focusing the optics. When considering these metrics, known systems have significant limitations.
One problem, for instance, is that repeatability, precision and accuracy can be severely compromised due to drift in the stage supporting the sample. Drift can occur during various phases of making AFM measurements, including during both set-up and operation. Notably, drift in this context is measured in nanometers/second. For conventional mechanical stages used in scanning probe microscopy, one to five nanometers/second of continuous drift is common. Clearly, if either the position of the tip or the position of the sample experiences drift before or during the measurement, an inaccurate measurement will be obtained. Drift affects the measurement in at least two ways, one being that repeating probe-sample positioning at a selected location on a line of the sample to be imaged, an important metric as understood in the art, is a challenge. Proving repeatability of the tool when drift is present can be nearly impossible. This becomes particularly challenging in view of the fact that there is enough variability in the line width that, if the tool is off by some fraction of the tip diameter when repeating the line measurement, a different measurement will be obtained.
The other primary reason data can be compromised by drift concerns line width variability. This problem is directly related to a metric that is monitored in semiconductor fabrication known as linewidth roughness (LWR), a measure of the variability of the width of the line itself.
With current SPM systems, as a line of a sample is scanned at a number of different places, different measurements of line width are obtained depending on the LWR. This is illustrated in FIG. 1. FIG. 1 illustrates a single line 10 having a width varying in what is shown as the vertical or “Y” direction. Line width roughness or LWR is essentially the deviation from the average of the independent widths, W1, W2, W3, etc. Depending upon the location at which the tip contacts the sample, different data will result. In the context of drift, the apparent width of line 10 will be expanded if drift occurs in a direction of scanning, and narrowed when drift is opposite the direction of scanning. As a result, a component of LWR will be introduced that is not due to the line itself but to drift. In many cases, this is the largest impact on the data due to drift, even more so than position repeatability along the line, since it is used to construct sample surface images.
Notably, it is only with the recent advancements in the resolution of scanning probe microscopy that LWR can even be measured and accounted for using SPM. In many known systems, users would not know that a different measurement was being conducted because the data would be essentially the same, requiring semiconductor manufacturers to use tools such as an SEM, and its attendant drawbacks, as understood in the art, to perform such measurements.
Known attempted solutions to the problem of controlling drift of AFM sample stages include providing an air bearing stage with a lock-down scheme, such as a vacuum lock down stage. However, even though such stages can be effective in minimizing the effects of drift, none of these systems correct for the position error that occurs during the lock down operation.
More particularly, current air-bearing stage technology allows for precise translation and a final position lock during which the air bearing is de-activated, most typically by applying a magnetic or vacuum force. However, the tradeoff with the benefits of an air-bearing stage (e.g., minimal adverse effects due to drift, fine positioning substantially free of counteracting forces, etc.) is that the lock down operation contributes to a final position error. When lock down of the stage occurs at a commanded or target position (using vacuum or magnetic force, or even gravity, for instance), Coulomb welding between the two pieces of the stage occurs so that the whole system responds like a solid piece of material. This often causes at least a micron or two of position shift of the stage. Moreover, the stage will oftentimes tilt during lock down, further compromising the precise positioning required for the applications contemplated by the preferred embodiments.
As a result, what was desired in the field of making atomic force microscope (AFM) measurements, particularly in the semiconductor industry, was an improved stage and corresponding method that minimizes positioning errors (e.g., due to stage lock down and drift), including improving position repeatability and reproducibility along AFM scan lines, as well as achieving a linewidth roughness (LWR) repeatability that yields increased throughput for high volume applications, such as semiconductor wafer measurement.